1. Field of the Invention
This invention relates to metallic diaphragms for compressors, and more particularly to provisions for increasing diaphragm life by reducing the failure incidence arising from the results of fretting and stress concentrations on the surface of the diaphragms.
2. Description of the Prior Art
The use of metallic diaphragms in non-lube diaphragm compressors offers many advantages over the use of elastomeric or plastic diaphragms in that higher pressures can be attained, higher temperatures can be accommodated, and less deformation is experienced. However, metallic diaphragms are more prone to failure from stress concentrations, which may occur due to dirt, chips or accretion of foreign material such as dust or extremely fine particles which become trapped between the diaphragm and the head of the compressor. The outer edge of the diaphragm cavity is the region where the combined stresses of bending moment and diaphragm stress occur, and where most diaphragms fail. Rarely do failures occur towards the center of the diaphragm, even though the movement is largest at the center.
Another phenomenon which appears at the outer edge of the cavity is that of fretting, which is caused by the sliding action of the dry metallic diaphragm against the dry metallic head. Chemical analysis has shown this to be oxide of iron in the case where the materials of construction for the head and/or diaphragm contain iron (such as any of the austenitic stainless steels, i.e. 304, 301, 316, etc.). This oxide slowly builds up to a point where the shape of the cavity is altered, increasing the bending moment which ultimately results in diaphragm failure at the outer edge.
It has also been observed that diaphragm failure occurs in the fretted area due to small particles of metal being "picked" out of the diaphragm. This "picking" action is probably the result of "cold welding" of the diaphragm to the head.
There has been no way yet developed to dramatically reduce or eliminate this fretting action, many solutions have been proposed such as hard facing the mating parts, plating, nitriding, and surface treatments of the heads and diaphragms, to no avail. The problem, of course, is accentuated by the presence of dust in the gas, which even with most careful gas filtration, still builds up over long periods of time.
Frankenberg in U.S. Pat. No. 1,712,657 discloses the use of radial grooves which provide a path for gas to exit from the head, and also discloses using one or more wide concentric stress risers in communication with the radial grooves to also distribute the gas pressure, but does not disclose grooves adjacent the outer edge of the chamber. The use of wide concentric stress risers in a high pressure compressor would result in considerable loss of compressor efficiency due to the dead space provided by the wide concentric stress risers.
Bowen in U.S. Pat. No. 3,668,978 discloses a typical compressor head with diaphragm for high pressure pumps, but contains no provisions for dealing with the problems of fretting and stress concentration.
My invention provides a storage place for the products of the fretting action and normal particles carried by the gas, to be accommodated, thereby reducing failure due to stress concentrations, and greatly extending diaphragm life.